According to a known continuously variable transmission device using a pair of pulleys and an endless V-belt, each pulley comprises a pair of members having coaxial and mutually opposing conical surfaces which are supported opposite to each other in a mutally axially slidable manner so as to define a V-groove whose width can be varied by relative axial motion of the two members. By thus varying the effective radius of the two pulleys in a corresponding manner, torque or rotational power can be transmitted between the two pulleys by the endless V-belt at a continuously variable transmission ratio.
In such a continuously variable transmission device, the endless V-belt is subjected to both repeated flexing and considerable tension as it travels between the two pulleys, and, in order to assure power transmission efficiency and reliability, the V-belt must be both flexible and durable. V-belts using synthetic resin, leather and other flexible materials may be sufficiently flexible for efficient power transmission but will wear out in a relatively short time when the power to be transmitted is great. Metallic links such as chains could be used for such a V-belt, but the lack of flexibility and the uneven contact between the belt and the pulleys would give rise to the problems of low power transmission efficienty and noise.
Japanese Patent Application No. 58-70920 (Japanese Patent Laying-Open Publication No. 59-197641) filed by one of the Assignees of the present application proposes a continuously variable transmission device which uses an endless V-belt comprising a plurality of of blocks having a V-shaped cross-section and a laminated metallic belt which is passed through the blocks and formed into a loop. Since the laminated metallic belt is sufficiently flexible to assure high power transmission efficiency and has a sufficient tensile strength to endure the tension applied thereto in transmitting power while the metallic blocks assure uniform contact between the V-belt and the pulleys and can transmit compressive force by their mutual contact, very favorable overall results can be obtained. However, it has been found that when such a continuously variable transmission device is subjected to severe load conditions for an extended time period, the laminated metallic belt tends to suffer uneven wear, particularly along the side edges.
The metallic tape or band used for forming such a laminated belt is required to be extremely thin, on the order of 0.1 mm to 0.2 mm, in order to be sufficiently durable against repeated bending stress, to be flexible, and contains a sufficient number of layers to endure the tensile stress. The material for such a metallic band is selected from quenched and annealed steel such as AISI 4340, precipitation hardening semi-austenitic stainless steel such as 17-7PH stainless steel, precipitation hardening martensitic stainless steel and maraging steel. The former two materials are relatively poor in toughness, while the latter two materials are relatively great in toughness and mechanical strength and are adapted to welding, but relatively poor in wear resistance.
The material for the metallic blocks is required to have high surface hardness and to be wear resistant since the blocks transmit torque by frictionally engaging with V-groove pulleys under severe load condition.
When a laminated metallic belt travels around the pulleys, relative velocity differences arise between the metallic band layers and between the laminated metallic belt and the metallic blocks, and the lateral side edges of the metallic band layers and the innermost layer of the metallic band are subjected to relative slips while receiving high surface pressure. The surface pressure tends to be high only because the metallic band is thin but also because the side edges of the metallic band are chamfered for the purpose of reducing stress concentrations in the corners. As a result, the lateral side edges of the metallic band tend to wear out faster than other areas and the durability of the metallic belt is impaired accordingly.
A nitriding process is well known as a means for improving the wear resistance and the fatigue resistance of metallic materials, but it is also known that if a nitriding process is attempted on an elongated member such as a metallic band for an endless laminated belt, problems arise because the dispersion of nitrogen in the metallic material causes dimensional increases and twisting deformation. Furthermore, a gas nitriding process will produce a brittle layer on the surface which must be removed by grinding. Tin plating and nickel plating may be used as an alternative to nitriding, but plating has the disadvantages of causing dimensional changes when performed on elongated thin band material. Plating is also costly.